Several processes have been developed for producing aggregate from fly ash. For example, Japanese patent 76015532-B describes the production of a lightweight aggregate from papermaking sludge and fly ash, kraft pulp, lime sludge, waste clay or earth. However, the resulting product had to be calcined at about 1100.degree. C., increasing energy requirements and costs. An article in Tokyo Kogyo Shikensho Hokoku (TKSHAI), Volume 63(1), pages 1-50, 1968, also describes aggregate formed from fly ash and paper-pulp waste. In this case, pulp waste liquor, i.e., liquid waste, was mixed with the fly ash.
European patent EP 430 232-A (priority U.5. SN 443,156) describes ceramic prepared by forming a suspension of clay and water, adding ash and organic matter and dewatering to below 60% by weight, shaping the particles and firing at elevated temperature. The ash, however, is obtained from sewage sludge or paper containing 10% clay and the organic matter is papermill waste or sewage sludge.
Japanese patent 02283678-A describes the production of ceramic products by mixing or kneading silica fines with industrial waste, such as sewage sludge (ash), sludge from water treatment plants, crude refuse incinerated ash, crushed waste soil, fly ash, paper sludge, pulp waste liquid, and sediment from the sea or rivers. The product is molded and fired at an elevated temperature.
These prior products have certain shortcomings. First, the energy requirements are substantial due to the requirement for firing the aggregate in an oven. On the other hand, if high temperatures are not used for calcining the product, organic matter will render the aggregate biologically unstable. For example, unless aggregates containing papermill waste with a significant organic content are calcined at a high temperature to destroy the organic compounds, the resulting product is biologically unstable. Organic breakdown products that are present create bad odors and can otherwise contaminate the environment by leaching from the product and can also cause inferior strength. One objective is to eliminate these problems without the need for oven drying.
Prior products have been "fused" as by being calcined in an oven. Consequently, they do not contain fiber, the fiber having been converted to ash in the oven. This removes any cohesive strength that could otherwise have been imparted to the product by retained fiber.
Finally, aggregates previously manufactured are not regarded to be environmentally stable with respect to the presence of heavy metals which can also leach from the product into the environment. The heavy metals that are primarily of concern in products of this kind are lead, mercury, chromium, strontium and cadmium. The need to prevent heavy metals from entering the environment is becoming increasingly important.
In view of these and other shortcomings of the prior art, it is a primary object of the invention to provide a substantially environmentally stable product from ash, especially coal ash either with or without the addition of papermill waste such as papermill sludge containing, for example, a mixture of paper fibers, clay and calcium carbonate.
Another object of the invention is to provide a biologically stable product from waste containing fly ash or bottom ash which can be dried at room temperature (e.g., about 50.degree. F.-90.degree. F.) rather than oven drying or calcining, thereby reducing energy requirements and yet producing an end product that will not contaminate the environment or emit bad odors.
Still another object is to provide an agglomerate or aggregate that can be used as a construction material and is lighter in weight than the standard aggregate commonly employed in concrete.
These and other more detailed and specific objects of the invention will be apparent in view of the following specification which illustrates by way of example but a few of the various forms of the present invention that will be apparent to those skilled in the art within the scope of the appended claims.